fix: strip ANSI at the source — clean terminal output before it reaches the model

Root cause: terminal_tool, execute_code, and process_registry returned raw
subprocess output with ANSI escape sequences intact. The model saw these
in tool results and copied them into file writes.

Previous fix (PR #2532) stripped ANSI at the write point in file_tools.py,
but this was a band-aid — regex on file content risks corrupting legitimate
content, and doesn't prevent ANSI from wasting tokens in the model context.

Source-level fix:
- New tools/ansi_strip.py with comprehensive ECMA-48 regex covering CSI
  (incl. private-mode, colon-separated, intermediate bytes), OSC (both
  terminators), DCS/SOS/PM/APC strings, Fp/Fe/Fs/nF escapes, 8-bit C1
- terminal_tool.py: strip output before returning to model
- code_execution_tool.py: strip stdout/stderr before returning
- process_registry.py: strip output in poll/read_log/wait
- file_tools.py: remove _strip_ansi band-aid (no longer needed)

Verified: `ls --color=always` output returned as clean text to model,
file written from that output contains zero ESC bytes.

tools/ansi_strip.py

@@ -0,0 +1,44 @@
+"""Strip ANSI escape sequences from subprocess output.
+
+Used by terminal_tool, code_execution_tool, and process_registry to clean
+command output before returning it to the model.  This prevents ANSI codes
+from entering the model's context — which is the root cause of models
+copying escape sequences into file writes.
+
+Covers the full ECMA-48 spec: CSI (including private-mode ``?`` prefix,
+colon-separated params, intermediate bytes), OSC (BEL and ST terminators),
+DCS/SOS/PM/APC string sequences, nF multi-byte escapes, Fp/Fe/Fs
+single-byte escapes, and 8-bit C1 control characters.
+"""
+
+import re
+
+_ANSI_ESCAPE_RE = re.compile(
+    r"\x1b"
+    r"(?:"
+        r"\[[\x30-\x3f]*[\x20-\x2f]*[\x40-\x7e]"     # CSI sequence
+        r"|\][\s\S]*?(?:\x07|\x1b\\)"                  # OSC (BEL or ST terminator)
+        r"|[PX^_][\s\S]*?(?:\x1b\\)"                   # DCS/SOS/PM/APC strings
+        r"|[\x20-\x2f]+[\x30-\x7e]"                    # nF escape sequences
+        r"|[\x30-\x7e]"                                 # Fp/Fe/Fs single-byte
+    r")"
+    r"|\x9b[\x30-\x3f]*[\x20-\x2f]*[\x40-\x7e]"       # 8-bit CSI
+    r"|\x9d[\s\S]*?(?:\x07|\x9c)"                       # 8-bit OSC
+    r"|[\x80-\x9f]",                                    # Other 8-bit C1 controls
+    re.DOTALL,
+)
+
+# Fast-path check — skip full regex when no escape-like bytes are present.
+_HAS_ESCAPE = re.compile(r"[\x1b\x80-\x9f]")
+
+
+def strip_ansi(text: str) -> str:
+    """Remove ANSI escape sequences from text.
+
+    Returns the input unchanged (fast path) when no ESC or C1 bytes are
+    present.  Safe to call on any string — clean text passes through
+    with negligible overhead.
+    """
+    if not text or not _HAS_ESCAPE.search(text):
+        return text
+    return _ANSI_ESCAPE_RE.sub("", text)